1. Field of the Invention
The present invention relates to printing machines in general.
More particularly, the invention relates to stencil printers.
Still more specifically, the invention relates to a stencil printer of the type wherein a series of frame-mounted stencils is seriatim lowered into a position for printing onto a workpiece.
2. The Prior Art
Stencil printers using frame-mounted printing stencils (flat printing screens) are known. For example, German allowed application DT-AS No. 1,252,167 discloses a stencil printer in which the workpiece to be printed is intermittently advanced past a printing station. After every advancing step a stencil is lowered onto the workpiece, printing ink or another medium to be applied by screen printing is admitted onto the upper surface of the stencil, and a squeegee then squeezes the medium through the stencil perforations and onto the workpiece.
If machines of this type print only with a single color, then only one printing stencil is required. If, however, multi-color printing is required, then the machines may be provided with a plurality of printing stations (each having a stencil for a different color) which are arranged one behind the other along the path of movement of the workpiece. A disadvantage of such an arrangement is that multi-color printers of this type must be very long since each of the printing stations requires a substantial amount of space. This is particularly pronounced when a pattern to be printed requires a very complicated color spectrum, in which case up to twelve printing stations, each with its own stencil, may have to be arranged one behind the other. The machine must thus be so extraordinarily long that a great many potential users simply do not have adequate space to install it.
In addition, there is a further drawback to the use of such long machines, namely the fact that when the workpiece is transported over such long distances, it tends to stretch out of shape--a particular problem with heavy workpieces such as yard-goods carpeting--with the result that pattern rapport from station to station is destroyed.
To overcome these problems it has been proposed to mount the series of stencils required for multi-color printing in a stacking frame at a single printing station, and to provide an arrangement for lowering the respectively lowermost stencil relative to the stacking frame and into printing position. This certainly reduces the overall length of the printer. However, it has two other distinct disadvantages, in that the stacking frame and the associated screen lowering mechanism are complicated and therefore expensive and prone to malfunction, and further that the predetermined printing sequence of the stencils in the stacking frame can be altered and/or any of the stencils in the stacking frame can be extracted or exchanged, only with considerable difficulty.